Turkey, which boasts the second-largest army in NATO, announced last week, on Tuesday, it had inked a deal to buy an S-400 (NATO version SA-21 Growler) missile defence system for $2-3 billion from Russia. The deal led NATO members to question Turkey's move and what it means for their alliance with NATO. The parties did not share the details of the S-400 agreement due to its sensitivity. But according to initial agreements, next year Turkey will receive two S-400 missile batteries from Russia and in the next two years will produce two batteries on Turkish soil. The agreement dates back to November 2016, however, at the end of April 2017, Turkish Foreign Minister Mevlut Cavusoglu made it clear that they were in talks with Russia regarding the S-400s and negotiations about joint production and costs have picked up steam since then.